How would parliament look under Proportional Representation?

In this year’s General Election, the Conservative Party only won 42 per cent of the overall vote, compared with Labour’s 40 per cent.

But they ended up with 318 parliamentary seats, while Labour only won 262 seats.

Meanwhile, the SNP got around twice as many votes as the Green Party. But the Greens still only have one MP, compared to the SNP’s 35 MPs.

This disparity is due to the voting system we use, known as First Past The Post (FPTP). It means that, in each constituency, the winner takes all – and votes for other candidates are effectively discounted.

So even if smaller parties win a significant share of the overall national vote share, they could still end up with no MPs if they have failed to come out on top within any single constituency.

Technically it’s still one person, one vote. But some parties are more likely to see these votes translate into MPs than others, depending on the distribution and concentration of their supporters.

The UK is the only undisputed democracy in Europe to use FPTP; others use systems that better reflect the overall proportion of votes cast, rather than just who wins in each constituency.

So how would the 2017 General Election have played out under proportional representation?

By combining election data with the results of a large-scale opinion poll, the Electoral Reform Society has this week published projected results.

They have looked at three different versions of proportional representation, which are explained below: Alternative Vote (AV), Additional Member System (AMS) and the Single Transferable Vote system (STV).

It’s important to remember that these are only estimates. It’s impossible to know exactly how voting behaviour might change if the UK switched to proportional voting – or how parties might alter their campaign tactics.

However, it gives us an idea of what the political map may look under proportional representation.

It suggests that the current FPTP system is only beneficial for the SNP and Conservatives, who would both lose out if the voting system was changed. Plaid Cymru would not stand to gain from a change either.

But the other parties would all benefit from switching to a version of proportional representation because second preferences now become relevant.

However, the different types of proportional representation are better for different parties, which perhaps makes it more unlikely that they would ever agree on a new system.

The Lib Dems, the Green Party and Ukip would all fare best under AMS. But this would actually be worse for Labour than the current method.

However, under STV – the version generally regarded as the most representative proportional voting system – Labour would have won more seats than any other party.

The voting systems explained

First Past The Post (FPTP) – This is the system currently used in UK general elections. Each voter picks a single candidate and the person with the most votes wins. They could get one per cent of the votes, or 100 per cent – it doesn’t matter. Just so long as they get more than their rivals.

Alternative Vote (AV) – Instead of just voting for one candidate in your constituency, you can rank them in order of preference. If any candidate gets more than 50 per cent of the first choices, they win. If not, the second preferences are added into the mix – and so on, until a candidate reaches the threshold.

Additional Member System (AMS) – This is a mixed method, which is already used to elect the Scottish Parliament, Welsh Assembly and London Assembly. You vote twice: once for a constituency representative, and once for a regional one. The constituency vote uses the FPTP system, while the makeup of regional members reflects the proportion of votes cast for each party.

Single Transferable Vote (STV) – This is possibly the most complex system, but it is also the one most strongly advocated by the Electoral Reform Society and is already used in certain elections in Scotland and Northern Ireland. Instead of electing one MP for each constituency, voters pool together to elect a small team of MPs to represent a wider region. Like with AV, voters rank the candidates in order of preference. To win, they must reach a pre-set quota. If no one reaches this quota level, the candidate with the least votes is eliminated and their votes are redistributed, until someone reaches the quota. Campaigners say this is the closest to true proportional representation of any voting system currently in use.